Various electric security devices are known. The known devices however, are relatively bulky and expensive. One type of such known security devices protects the security of all the machines in a shop. The problem with this type of device is that a single defective electric machine will cause the security device to stop all the machines in the shop, irrespective of whether they are defective or not. Other types of known security devices are connected between a power line and ground. But if the grounding wire itself becomes defective, then the fault will not be detected.